1. Field of the Invention
The present invention relates t o an X-ray fluorescence inspection apparatus which uses fluorescent X-rays to inspect the inclusion amount of a contaminant such as tungsten in a measurement portion of a material being inspected, such as in a pressure welded portion of a fuel rod for example.
2. Description of the Related Art
Heretofore, in the production of fuel rods for a nuclear fuel assembly for use in a light water nuclear reactor for example, as described in Japanese Patent Application, First Publication No. Hei-6-18699, a lower end plug is forced into one end of a cladding tube packed with a plurality of pellets, and a coil spring is then inserted from the other end and an upper end plug fitted, after which both ends are girth welded. An inert gas is then forced into the inside of the cladding tube via a seal aperture formed in a central region of the upper end plug, and the tube then sealed by pressure welding the seal aperture.
For example in FIG. 9, which is a partial cross-sectional view showing the upper end plug fitted into the cladding tube, the pressure welding is conducted inside a sealed chamber. That is, an end portion 2a of a cladding tube 2 of a fuel rod 1 is positioned inside a chamber (not shown in the figure), and an upper end plug 3 is then forced into the end portion 2a of the cladding tube and girth welded. The upper end plug 3 incorporates a seal aperture 4 which passes through a central portion of an end surface 3a of the end plug 3.
Then an inert gas such as helium is forced in through the seal aperture 4, and with the system under pressure, arc welding from a tungsten electrode 5 is then used to seal weld the seal aperture 4. The arc discharge causes the periphery of the seal aperture 4 of the end plug 3 to melt, thus seal welding the aperture 4 shut. Following completion of the welding of the aperture 4, then as shown in FIG. 10 a bead portion 7 (welded portion) is produced in the center of the end surface 3a by pressure welding.
However, since for reasons of corrosion resistance and neutron economy, the end plug 3 of the fuel rod 1 is made of zircaloy, then when the seal aperture 4 of the end plug 3 is pressure welded using a tungsten electrode 5, because a large voltage is applied, a phenomenon can occur where the tungsten of the tungsten electrode 5 melts and is splashed into and mixed with the melted bead portion 7 of the end plug 3. This is known as tungsten inclusion. Mixing of tungsten with the zircaloy of the end plug 3 is undesirable as it causes a deterioration in the corrosion resistance of the end plug 3.
Consequently, heretofore the end plugs 3 of fuel rods 1 are inspected for tungsten inclusions, and defective end plugs with a large amounts of inclusions and poor corrosion resistance are detected.
Examples of inspection apparatus for inspecting the amount of tungsten inclusions include apparatus based on X-ray through transmission methods. With this inspection method X-rays are irradiated from an X-ray generation apparatus, and are transmitted through the end plug 3 to produce an X-ray projection on a film. In those cases where there are tungsten inclusions within the bead portion 7 of the end plug 3, because the tungsten is almost impenetrable to X-rays, white portions appear on the image, enabling detection of the tungsten inclusions as well as a determination of the amount of inclusions.
However, this inspection method requires the developing of a film, which can take time. Moreover the tungsten is detected by an examiner viewing the film, which requires experienced personnel.
X-ray fluorescence inspection apparatus have been proposed as inspection apparatus which can help overcome the above problems. These X-ray fluorescence inspection apparatus include both large scale stationary apparatus a s well as small scale portable types.
The large stationary types of apparatus use a method whereby X-rays are irradiated down on to the material being inspected, the X-ray output being large and the precision high. However, whereas the bead portion 7 being inspected is of an approximate diameter of 3 mm, the fuel rod 1 is a cylinder of total length 4 m and diameter 10 mm. In order to inspect a plurality of this type of fuel rod 1 on line, it is preferable if the fuel rods 1 are arranged horizontally with the X-rays being irradiated in a horizontal direction. This orientation however is not possible with a large stationary X-ray fluorescence inspection apparatus.
On the other hand, with the portable types of X-ray fluorescence inspection apparatus, horizontal al irradiation of the X-rays is possible, as shown in FIG. 11 for example.
With the X-ray fluorescence inspection apparatus shown in FIG. 11, a holder 15 constructed of steel or the like, is mounted around the circumference of the end plug 3 of the fuel rod 1, with a surface 15a of the holder 15 approximately parallel with the end surface 3a of the end plug 3. A portable X-ray fluorescence inspection device 16 is then positioned facing the end surface 3a of the end plug 3.
The portable X-ray fluorescence inspection device 16 irradiates primary X-rays from an X-ray tube 17 in an approximately horizontal direction toward the bead portion 7 of the end surface 3a. A fluorescent X-ray detection apparatus 18 then detects and measures fluorescent secondary X-rays generated at the end surface 3a of the bead portion 7.
The measurement principles relating to this type of X-ray fluorescence inspection device 16 will now be explained with reference to FIG. 12.
Primary X-rays are irradiated from the X-ray tube 17 at the bead portion 7 of the end surface 3a of the end plug 3, exciting elements included within the bead portion 7. The excited elements then produce fluorescent X-rays of a specific wavelength in the form of secondary X-rays. The fluorescent X-rays pass through a divergence slit 19 in the detection apparatus 18 and are directed at an analyzing crystal 20 where e diffraction into an X-ray spectrum occurs for each wavelength, at an angle .theta. determined by the Bragg formula shown below. EQU n.lambda.=2d sin .theta.
where n is the degree (n=1,2,3 . . . ), .lambda. is the wavelength (.ANG.), d is the spacing of the lattice planes of the crystal, and .theta. is the diffraction angle (.degree.)
Then, each fluorescent X-ray spectrum passes through a light intercepting slit 21 and is detected by a detector 22 and measured.
The analyzing crystal 20, the detector 22 and the light intercepting slit 21 are configured as shown in FIG. 12 to enable scanning to be conducted while maintaining the double angle relationship (.theta.-2.theta.) with respect to the fluorescent X-rays being irradiated on to the analyzing crystal 20. Consequently by scanning the analyzing crystal 20, the detector 22 and the light intercepting slit 21 across the entire angle range (13.degree..about.98.degree.), the fluorescent X-rays of specific elements can be detected at specific angles (wavelengths). The fluorescent X-rays detected are converted to a voltage pulse by the detector 22 and are then sent to either a scaler (scaling circuit) or a rate meter, which are not shown in the diagram, and the X-ray intensity then displayed as a coefficient.
The wavelength of the fluorescent X-ray is specific for each element and can be used for identification of element s such as tungsten. The X-ray intensity at each wavelength is proportional to the amount of that element contained in the sample material.
The portable X-ray fluorescence inspection device 16 however does not provide an apparatus for the simple adjustment of the primary X-ray irradiation range (the measurement range), and because the X-ray beam diffuses in the direction of the irradiation, using a stainless steel holder 15 as a positioning jig will generate unwanted fluorescent X-rays from the holder 15 which will appear as noise at the detection apparatus 18 resulting in a lowering of the accuracy of the inspection.
Furthermore, the fluorescent X-ray spectra Zr-K.alpha., Zr-K.beta. resulting from excitation of the zircaloy of that portion of the end surface 3a of the end plug 3 surrounding the bead portion 7 is also detected by the detection apparatus 18, and because a multiple of the wavelength of the spectra Zr-K.alpha., Zr-K.beta. is very similar to the wavelength of the fluorescent X-ray spectrum for tungsten W-L.alpha., the spectra Zr-K.alpha., Zr-K.beta. are detected by the detector 22 as noise together with the tungsten fluorescent X-rays, and as the amount of noise is considerable an accurate determination of the amount of tungsten inclusion cannot be made. Furthermore, although measurement over an extended time period produces an improvement in the accuracy of the inspection, this results in a deleterious reduction in the productivity of the inspection.
Moreover, in the case of fuel rods with differing external dimensions, even if the various beads 7 contain the same proportion of tungsten, because the dimensions, shape and surface area of the end surfaces 3a of the end plugs 3 will be different, the amount of fluorescent X-ray radiation detected by the detector 22 will also be different, and so the measured values of the fluorescent X-rays detected will differ. Consequently, the complication arises that a different calibration curve (inspections standard) needs to be provided for each type of fuel rod 1 in order to determine whether or not the product is of an acceptable standard.